The present invention relates in general to magnetic field sensors and, more particularly, to magnetoresistive sensors.
Magnetoresistive sensors are typically small, and generally measure magnetic fields on the order of 0.001 gauss to 100 gauss. Also, magnetoresistive sensors are able to measure D.C. fields as well as fields having frequencies up to and exceeding one megahertz. Accordingly, magnetoresistive sensors are used in a wide variety of applications such as current sensing, proximity sensing, etc.
The magnetoresistive material used in making magnetoresistive sensors is a material whose resistance changes in the presence of a magnetic field. Permalloy, which is a nickle/iron alloy, is such a material and is often provided as a film in a layer above an integrated circuit wafer. The resistance of the film varies according to the square of the cosine of the angle between the magnetization direction of the film and the direction of the current running along the length of the film. When the magnetization of the film is parallel to the current, the resistance of the film is at a maximum. On the other hand, when the magnetization of the film is perpendicular to the current, the resistance of the film is at a minimum.
The response of a magnetoresistive material is measured as xcex94R/RN, where xcex94R is the change in resistance of the magnetoresistive material and RN is the nominal resistance of the magnetoresistive material. The change in the resistance xcex94R of Permalloy between the point where the magnetization direction is parallel to the current direction and the point where the magnetization direction is perpendicular to the current direction is typically on the order of 2% of the nominal resistance of the material.
Moreover, the plot of xcex94R/RN versus the angle between the magnetization direction and the current direction is bell shaped. In order to operate the magnetoresistive material on the linear part of this curve, a bias field is frequently applied to the magnetoresistive sensor. For example, either a solenoid wrapped around the magnetoresistive sensor package or a plurality of thin-film permanent magnets at the end of the magnetoresistive sensor are usually used to apply an external biasing field so as to bias the magnetoresistive material at this linear portion.
Alternatively, instead of applying an external biasing field, it is known to apply an internal biasing field to the magnetoresistive sensor. Accordingly, the magnetoresistive sensor is provided with a conductive strap, which is usually referred to as a set/reset strap. A set-reset strap is fabricated using known integrated circuit processing techniques to form a serpentine conductor typically in a layer above the magnetoresistive film. A current may be applied in either direction through the set/reset strap so as to selectively bias the magnetization direction of the magnetoresistive film.
This set/reset strap may also be used as an offset strap to eliminate the offset due to mismatched magnetoresistive bridge elements and due to temperature differences between magnetoresistive films when several magnetoresistive films are arranged in a bridge configuration in a single sensor structure. The offset strap can also be used to eliminate offset drift in the bridge measurement electronics.
As indicated above, known set, reset, and/or offset straps meander in a single plane or layer of a magnetic device such as a magnetoresistive sensor. Accordingly, when multiple magnetic devices are formed on a semiconductor wafer, a substantial amount of the wafer real estate is used to form the strap, which imposes a restriction on the number of magnetic devices that can be formed on the wafer. Moreover, known set, reset, and/or offset straps which meander in a single plane or layer of a wafer require a relatively large current flow to produce the required magnetic field.
The present invention is directed, at least in one embodiment, to a strap which overcomes one or more of the problems noted above.
In accordance with one aspect of the present invention, a magnetic sensor comprises a semiconductor substrate, a magnetically responsive material formed above the semiconductive substrate, and a conductive strap wound into a coil around the magnetically responsive material such that at least a portion of the conductive strap is between the magnetically responsive material and the substrate.
In accordance with another aspect of the present invention, a magnetoresistive sensor comprises a semiconductor substrate, an insulator over the substrate, a magnetoresistive film embedded in the insulator responsive material, and a conductive strap wound through the insulator so as to form a coil around the magnetoresistive film.
In accordance with yet another aspect of the present invention, a magnetoresistive sensor comprises a semiconductor substrate, a magnetoresistive material, and a three-dimensional conductive strap. The magnetoresistive material is formed above the semiconductive substrate. The three-dimensional conductive strap is formed above the semiconductive substrate, and has a position with respect to the magnetoresistive material so as to set the magnetization direction of the magnetoresistive material when the three-dimensional conductive strap is supplied with current.